In recent years, the technique of a light crystal display has been remarkably progressed, and has been widely used as a display device of a personal computer, a television, a cellphone or the like. Since these liquid crystal displays are such that a liquid crystal display unit alone has not the light emitting function, displaying becomes possible by mounting a back light unit on a back side thereof.
There are many modes in a back light unit, and these are roughly classified into two kinds. Generally, a most frequent mode is a mode called internal illumination mode or direct-type, in which a light source is inside an illumination surface. In this mode, since many light sources such as cold cathode-ray tubes and the like can be arranged immediately below an illumination surface, the mode has the characteristic that an extremely high luminance is obtained, and light loss is small as compared with an edge light mode described later. For this reason, a direct-type mode is frequently used in a liquid crystal display which is large-scale as in a large-scale liquid crystal TV and requires a high luminance.
However, in the direct-type mode, there is a problem that a great difference in a luminance is easily generated between a position which is immediately over a light source, and a position which is not, on a screen, and this is easily recognized as a luminance variation. For this reason, a light diffusion plate consisting of acryl or polycarbonate having a thickness of a few millimeters, in which a light scattering substance such as an organic or inorganic fine particle is mixed and, if necessary, a light diffusion film in which a surface of a biaxially stretched polyester film has been subjected to light diffusion processing are arranged on a light emitting surface, thereby, reduction in a luminance variation is tried.
The other mode is a mode called edge light-type, in which a light source is arranged outside an illumination surface, for example, a generally linear light emitting body such as a fluorescent lamp (cold cathode-ray discharge tube) is adhered to one side or two sides of a light guiding plate consisting of a transparent resin plate which is an illumination surface, and a lamp cover consisting of a reflecting body is disposed to introduce light into the light guiding plate. This mode has the characteristic that a consumed power is smaller, and miniaturization and thinning are possible, as compared with the direct-type back light unit. For this reason, particularly when thinning and weight saving are required, such as in a small display such as a notebook personal computer and the like, an edge light-type back light unit is widely used.
The necessary function which requires a light guiding plate of the edge light-type back light unit to have, includes the function of forwarding light introduced through an end part, and the function of sending the forwarded light on a liquid crystal display element side. The former function is determined depending on a material used, and interface reflection property, and the latter function is determined depending on a shape of a light guiding plate surface which avoids the total reflection condition. With respect to a shape of a light guiding plate surface for avoiding this total reflection condition, a method of forming a white diffusion material on the light guiding plate surface, and a method of forming a lenticular or prism frenel shape on the light guiding plate surface are known.
However, light emitted from the light guiding plates on which these shapes are formed has an uneven light distribution depending on the shape. Therefore, in order to obtain an image of high quality, one tries to make a luminance on a light emitting surface uniform by arranging a light diffusion film or the light guiding plate to diffuse and scatter light passing through a light diffusion layer.
In order to improve a front luminance, a condensing sheet is used in these back light units in some cases so as to collect emitting light having transmitted through the light diffusion film in a front direction as much as possible. This condensing sheet is a transparent sheet in which many fine irregularities such as a prism shape, a wave shape, a pyramid shape and the like are arranged on a surface thereof, and emitting light having transmitted through the light diffusion film is refracted to concentrate on a front surface to improve a luminance on an illumination surface. Such the condensing sheet is used, alone or by stacking two sheets, on a surface side of the light diffusion film.
Further, in order to make a luminance variation generated by disposing the condensing sheet, or a defect of the condensing sheet not remarkable, the light diffusion film is disposed also on a surface side of the condensing sheet in some cases.
And, in order to suppress loss of light to improve utilization effectiveness of light, a material having a high transmittance is required in each member (light diffusion plate, light guiding plate, light diffusion film, condensing sheet etc.) constituting the back light unit.
On the other hand, these members generally have a construction that a functional layer is laminated on a substrate film, and these members are converted into a composite by sticking them together via a pressure-sensitive adhesive. By reducing the number of supports of these members, the number of times light is reflected at an interface between members having different refractive indices, can be reduced. For this reason, it is also effective for enhancing a light utilization efficiency to reduce the number of members. On the other hand, trial to add other function (e.g., light diffusibility) to a single substrate film itself has been studied.
As the light diffusion film used in the back light unit, for example, a film obtained by molding a transparent thermoplastic resin into a sheet, and subjecting the sheet to processing of physically imparting irregularities to a surface, and a film obtained by coating a light diffusion layer consisting of a transparent resin containing fine particles on a surface of a biaxially stretched polyester film are disclosed (see e.g., Patent Literatures 1, 2).    Patent Literature 1: JP-A 4-27550    Patent Literature 2: JP-A 6-59108
Particularly, since the film obtained by coating a light diffusion layer consisting of a transparent resin containing fine particles on a film of a biaxially stretched polyester film has a high light transmittance, and has excellent light diffusibility, and has excellent heat resistance, mechanical strength and, further, excellent thickness uniformity which are characteristics of the biaxially stretched polyester film, it is widely adopted.
However, in this method, there is a problem that, when the light diffusion film is heated, curling is easily generated due to a difference in a linear expansion coefficient between a substrate film and a light diffusion layer. This problem is becoming an important problem particularly in a liquid crystal display adopting a direct-type back light unit, which is large and requires an extremely high luminance, such as a large liquid crystal TV in recent years. This is because as a light diffusion film has a larger area, an area of an interface between layers having different linear expression coefficients becomes larger, and curling becomes remarkable when the light diffusion film is heated. Further, as a display has a higher luminance, a consumed power of a light source, that is, a heating value of the back light unit becomes larger and, under such the situation, curling is more easily generated. In addition, in this method, a light diffusion layer is formed by post processing, and this is disadvantageous regarding a market's demand of a lower cost.
On the other hand, many trials to make a biaxially stretched polyester film itself have light diffusibility for the purpose of reduction in the cost have been proposed, such as reduction in the number of back light unit parts and simplification of a production process by integration with other optical functional film such as the light diffusion film and a condensing sheet. And, an approach of trying to make a biaxially stretched polyester film itself having excellent heat resistance and mechanical strength and, further, excellent thickness uniformity simultaneously have light diffusibility leads to solution of the problem of heat curling. Therefore, its industrial value is very great.
However, any of trials to make a biaxially stretched polyester film itself have light diffusibility which have previously been proposed, deteriorates any of characteristics intrinsically possessed by the biaxially stretched polyester film, or deteriorates property which should be possessed by the light diffusion film, such as light transmittance and light diffusibility, and those trials have not been put into practice. For example, a biaxially stretched polyester film consisting of a composite film of two or more layers in which at least one layer is a layer containing fine bubbles therein is disclosed (see e.g. Patent Literature 3)    Patent Literature 3: JP-A 11-268211
In this method, the film has characteristics intrinsically possessed by the biaxially stretched polyester film, such as excellent heat resistance and mechanical strength, and excellent thickness uniformity. However, light diffusibility is imparted by bubbles present inside the layer, there is a problem that a light transmittance is low. This is because since bubbles (voids) generated in a step of biaxially stretching a film have a plate-like form parallel with a film surface, when this is used as a light diffusion film in a back light unit, light emitted from an illumination surface is scattered back, deteriorating a light transmittance. Actually, a total light transmittance shown in Examples is only 83% at highest.
In addition, a multilayer-type biaxially stretched polyester film in which a substrate film consists of polyethylene terephthalate, and a low melting point polyester resin having a melting point of 200° C. or lower is used as a resin constituting a light diffusion layer is disclosed (see e.g. Patent Literature 4).    Patent Literature 4: JP-A 2001-272508
In the above method, suppression of voids appearing around a light diffusing agent and hampering transparency is taken into consideration. For this reason, balance between light transmittance and light diffusibility is comparable to that of the previous light diffusion film obtained by coating a light diffusion layer consisting of a transparent resin containing fine particles on a biaxially stretched polyester film.
However, in the light diffusion film obtained by the methods described in Patent Literature 4, there is a great difference in a melting point between a polyester resin constituting the light diffusion layer and a polyester resin constituting the light diffusion layer. As a result, since the resulting biaxially stretched film has a linear expansion coefficient different between the light diffusion layer and the substrate layer, the biaxially stretched polyester film itself becomes easy to be curled at heat treatment. For this reason, curling is generated by heat treatment at a post-processing step in some cases, or curling is generated in some cases depending on environment (temperature) under which a liquid crystal display is used, and there is a possibility that a luminance at a light emitting surface in a back light unit becomes ununiform.
In addition, there is disclosed a film in which a copolymerized polyester or an amorphous polyester, each having a melting point of 210° C. or lower, is used as a constituent resin, a light diffusion layer obtained by incorporating a light diffusion additive consisting of an incompatible particle or a thermoplastic resin in the constituent resin is disposed as an intermediate layer, and a crystalline polyester resin layer forming a smooth surface, consisting of polyethylene terephthalate, is laminated on both sides thereof (see e.g. Patent Literatures 5-11).    Patent Literature 5: JP-A 2001-324605    Patent Literature 6: JP-A 2002-162508    Patent Literature 7: JP-A 2002-182013    Patent Literature 8: JP-A 2002-196113    Patent Literature 9: JP-A 2002-372606    Patent Literature 10: JP-A 2004-219438    Patent Literature 11: JP-A 2004-354558
In the above methods, unlike Patent Literature 4, since a film has a symmetrical structure, generation of curling due to an asymmetrical structure seen in the Patent Literature 4 is improved. However, since resins having greatly different melting points or crystallizabilities are used as a constituent component, there is a potential cause for increase in generation of curling due to variation in the production condition or the like. In addition, in the above methods, a layer consisting of a low melting point copolymerized polyester resin or an amorphous polyester rein accounts for 80% or more of a total thickness of a laminated film, excellent properties such as heat resistance, mechanical strength and thickness uniformity which are characteristics intrinsic to a crystalline biaxially stretched polyester film are deteriorated. As a result, remarkable dimensional change and deterioration of planarity are generated in processing at a high temperature or use under high temperature environment, and the original object of the light diffusion film of uniformizing a luminance at a light emitting surface in the back light unit can not be attained.
In addition, a biaxially stretched polyethylene terephthalate film incorporating spherical or convex lens-like particles of a specified particle diameter is disclosed (see e.g. Patent Literature 12).    Patent Literature 12: JP-A 2002-37898
In the above Literature, a film having a total light transmittance of 88% and a diffusion transmittance of 68%, using polyethylene terephthalate as a raw material of a polyester is disclosed in Example 1. In addition, a film having a total light transmittance of 85% and a diffusion transmittance of 63% is disclosed in Example 5. These light transmittances are an excellent physical property value comparable to those obtained by coating a light diffusion layer consisting of a transparent resin containing fine particles on a surface of the biaxially stretched polyester film.
However, fundamental properties such as heat resistance, mechanical strength, thickness accuracy and the like of these films are not disclosed at all, and a possibility of obtaining heat resistance, mechanical strength and high thickness accuracy which are characteristics intrinsic to the biaxially stretched polyethylene terephthalate film is not recognized at all. The reason is as follows:
Although these films are films obtained by stretching an unstretched film having a thickness of 200 μm at each stretching ratio of 3.0 in both longitudinal and transverse directions, that is, at an area ratio of 9.0, a thickness thereof is 50 μm, and an actual area stretching ratio calculated from a ratio of thickness before and after stretching is merely 4.0.
That is, it is thought that a set stretching ratio and an actual stretching ratio have been remarkably separated by influence of a distribution of width constriction generated at longitudinal stretching, and of a stretching ratio in a width direction generated at transverse stretching and, further, a dimensional change at heat treatment. And, by stretching at an actual area stretching ratio of around 4, even when excellent light transmittance is obtained, it is almost impossible to obtain heat resistance, mechanical strength and high thickness accuracy which are characteristics intrinsic to the biaxially stretched polyester film.
From the foregoing situation, the method of making the biaxially stretched film itself have light diffusibility is inferior to the method of post-processing a light diffusion layer on a transparent substrate film in such a comprehensive quality that both light transmittance and light diffusibility are realized while maintaining heat resistance, mechanical strength and high thickness accuracy which are characteristics intrinsic to the biaxially stretched film. For this reason, this method has not been put into practice.